Noise cancellation headphones provide the wearer with an ability to listen to sound free from the disturbing effects of background noise. Noise cancellation headphones are used widely in commercial passenger aircraft and general aviation and are now experiencing adoption in the mainstream in a variety of consumer audio applications.
Headphones, whether passive or noise cancellation, can be designed in either a supra aural or circum aural configuration. In the case of the former, the headphone rests on top of the ear with the interface to the wearer typically being soft open-cell foam. In the case of the latter, the ear cup completely encloses the ear with the human-headphone interface typically being a foam based leatherette ear pad.
Noise cancellation headphones are also configured in circum aural or supra aural arrangements. Circum aural noise cancellation headphones, however, tend to provide a better overall noise suppression effect as the complete seal provided by the ear pad insulates the ear from the higher frequencies of sound which are more difficult to reduce by active noise cancellation techniques.
Headphones, whether passive or active noise cancellation, are typically large and comprise a headband that can either be worn on top of the head or behind the neck. Headphones can be clumsy, uncomfortable and space consuming, especially for those who travel frequently.
An alternative solution for the personal reproduction of sound is an earphone such as an “ear bud” which is placed directly in or adjacent to the auditory canal. Known earphones generally comprise one or two small audio transducers that are placed directly in or adjacent to the auditory canal. Earphones are used widely with hands-free cellular phone kits and portable audio devices such as mp3 and DVD players.
Earphones can be difficult to locate within the ear, leading to the user discomfort, and in some cases poor performance for the user. Incorrect fit can also lead to earphones falling from the user's ear.
Presently, few noise cancellation ear buds solutions exist in the marketplace. The few products that have been developed and commercialised rely on a feed forward active noise cancellation configuration.
A feed forward active noise cancellation system is relatively simple in that it relies on a reference signal to generate a control response; this reference signal being somehow related to the signal requiring control.
In the case of a feed forward earphone active noise cancellation solution, the best choice of reference signal is a measure of the ambient noise directly outside of the earphone's seal against the ear. This reference signal, obtained by way of a microphone transducer, is processed by noise cancellation electronic circuitry (filters) to generate an appropriate control response. The circuitry is designed to replicate the dynamic behaviour of the acoustic system between the reference measurement and control positions. All things being equal, the control response, once output via the earphone's speaker will effect cancellation of the noise that has infiltrated the ear canal. A feed forward controller is ‘dumb’ in the sense that it does not have any measure of its own performance. It relies on a prior knowledge of the disturbance (noise) and the acoustic system.
Unfortunately, in the case of a feed forward earphone active noise cancellation control configuration, the reference signal is not fully representative of the noise that actually penetrates the earphone's seal and enters the auditory canal. The maximum performance of a feed forward active noise cancellation system can be calculated mathematically by measuring the coherence between the reference signal and the sound that penetrates the ear canal. This can be significantly less than unity, especially where the ear bud does not present a tight seal around the ear canal or the acoustics of the ear canal varies from that measured to determine the control filters.
A feedback control configuration relies on an error measurement located downstream from the point of control. The error represents a logical difference between a desired outcome and the measured result.
As the control response of a feedback control configuration is directly related to its own output it is far more susceptible to an instability condition. This is especially true where the system under control is subject to change. In the context of active noise cancellation, instability manifests itself as an uncontrolled ringing. Such a condition is unpleasant and can damage the hearing organ. Instability problems have lead to very few earphones which incorporate active noise cancellation systems being successful, commercially viable, consumer products. Development of an effective active noise cancellation product requires a careful balancing of a number of system parameters.
Although some noise cancellation (e.g. less than 20 dB) can often be achieved, instability problems mean that effective noise cancellation using an earphone is very difficult without the need for a complex controller. For example, U.S. Pat. No. 4,985,925 discloses an earplug including a speaker and a microphone for use with an active noise reduction control circuit having a shunt feedback control filter. The earplug can for some audio frequencies provide effective noise cancellation, but a very complex control circuit is required. Also, despite the complex controller, active noise cancellation is poor between 1 kHz and 2 kHz, and the earplug may still have a stability problem in that frequency range.
Another constraint on a controller for present consumer earphone products is size, weight and power constraints. For example, when being used in conjunction with a portable MP3 player a controller needs to be small and light enough to be worn as a medallion, and typically needs to operate off a 1.5 to 3 volt battery power supply. Therefore the controller order, driving voltage swing and available power are very limited. This also makes it important to provide a solution that reduces the demands placed on the controller.